The invention includes a reciprocating-piston compressor having at least two working cylinders which are arranged in series and along a cylinder axis and in which in each case one piston is guided in an axially movable manner, with the pistons having a common axially actuated piston rod which extends through a passage opening in a partition between the working cylinders.
In the reciprocating-piston compressors known from the prior art, a contact seal in the form of a sealing ring is conventionally provided between the passage opening and the piston rod, in order to seal off the working cylinders, which are arranged in series, with respect to one another. In particular where reciprocating-piston compressors are used in compressed-air brake systems of utility vehicles such as commercial trucks, a high compressor power is required on account of the high compressed air demand, and the reciprocating-piston compressor must therefore perform a high number of compression strokes. The previously-used contact seals, however, generate friction, such that relatively high friction losses are generated as a result of the high number of compression strokes, which friction losses also manifest themselves in high temperatures of up to 300° C. in the region of the seal. For these reasons, a low-friction and simultaneously heat-resistant material is necessary for the seals, which is correspondingly expensive.
In contrast, the embodiments according to the present invention provide a reciprocating-piston compressor of the type specified above that can be produced more economically.
The invention embodiments provide for sealing off the working cylinders in the region of the piston rod exclusively by using a non-contact seal in the form of an axial gap seal which is formed between a radially outer circumferential surface of the piston rod and a radially inner circumferential surface of the passage bore. In other words, in one example the piston rod extends through the passage opening without the interposition of a separate contact seal. Although a certain degree of leakage then takes place between the working cylinders, the leakage however does not pose a problem in the exemplary design of reciprocating-piston air compressors with at least two working cylinders arranged in series, since each working cylinder is acted on with compressed air in any case. It is then possible for the previously conventional contact seals in the partitions between the working cylinders, which are associated with the disadvantages mentioned in the introduction, to be dispensed with.
The invention utilizes the viscosity properties of the air, on account of which compressed air has a reduced tendency to pass through a narrow gap in the event of a rapid pressure rise than in the event of a relatively slow pressure rise. Against the background of the high number of compression strokes per unit time, and thus the fast pressure rise in the working cylinders, which are conventional with reciprocating-piston air compressors in compressed-air brake systems of utility vehicles, a small amount of leakage is consequently to be expected.
The axially extending annular gap, between the radially outer circumferential surface of the piston rod and the radially inner circumferential surface of the passage bore, forms a throttle at which the gap flow loses pressure energy. The gap, depending on the gap width, consequently reduces a high pressure level to a significantly lower level as a result of the throttling.
It is particularly preferable, to form a labyrinth gap seal, for at least the radially inner circumferential surface of the passage bore to be provided with radial grooves which are arranged with an axial spacing to one another. In a labyrinth gap seal of that type, the fluid flows from a chamber of the one working cylinder, which is at relatively high pressure, into a chamber of the other working cylinder, which is at a relatively low pressure, through a plurality of constricted throttle points which are formed by the constrictions of the passage opening arranged between the grooves. The kinetic energy of the fluid flow is converted almost entirely into friction heat, that is to say into loss energy, in the widened spaces downstream of the throttle points, that is to say in the grooves.
In one preferred exemplary embodiment of the invention, the reciprocating-piston compressor is of reversing design, with the leakage flow which flows through the gap seal from the one working cylinder into the other working cylinder advantageously increasing the air volume which is to be compressed during the subsequent reversing movement of the piston rod.
In one exemplary refinement, the invention may also be applied to a multi-stage reciprocating-piston compressor which performs a multi-stage compression of the intake air and in which each working cylinder is assigned to a compression stage.
The invention is particularly preferably used, for the reasons already stated above, in a reciprocating-piston compressor of a compressed-air brake system of a utility vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.